The present invention relates to electrical terminals for connection to coaxial cable and more particularly relates to a terminal having a two-piece ground shield.
A typical coaxial cable for signal transmission includes a center conductive core or inner conductor surrounded by an inner dielectric sheath. An outer conductor or conductive shield surrounds the inner dielectric sheath and typically is a metal braid. The conductive shield is surrounded by an outer dielectric sheath of the cable.
Electrical terminals are known for terminating a coaxial cable so that the cable can be connected to another connecting device, such as a complementary mating connector, a printed circuit board, and the like. Such terminals typically include a core conductor or contact for mechanically and electrically connecting to an end of the inner conductor of the cable, a dielectric sleeve therearound, and an outer conductor or ground shield for mechanically and electrically connecting to the outer conductor of the cable.
The terminal must be configured and connected to the coaxial cable in such a manner as to minimize voltage standing wave ratio (VSWR), insertion losses, and radio frequency (RF) leakage. Commonly, the largest component of insertion loss is reflection due to impedance discontinuities. Energy reflected at impedance discontinuities does not reach the load. Such discontinuities result from variations in the radial distance between the outer surface of the inner conductor and the inner surface of the outer conductor. Such discontinuities include changes in diameter in either conductor or deformation of either conductor. RF leakage results from radiation loss through holes or seams in the terminal or cable.
Electrical and mechanical connection of the core conductor of the terminal to the inner conductor of the cable is commonly established by axially inserting the inner conductor into the core conductor and then radially crimping and/or to soldering the core conductor of the terminal to the inner conductor of the cable. It is also common for such electrical and mechanical connection of the ground shield to the outer conductor of the cable to be established by a method that includes stripping the end of the coaxial cable, sliding a ferrule onto the cable, folding the outer conductor braiding back over the outer dielectric sheath, inserting the stripped end of the coaxial cable into the ground shield, unfolding the braiding over the ground shield, sliding the ferrule over the braiding and ground shield, then crimping the ferrule to the braiding thereby pressing the braiding against the ground shield.
U.S. Pat. No. 3,854,003 to Duret discloses a ground shield terminal assembly comprising two elastic half-shell conductive housing members having chamfered ends. The assembled housing members are surrounded by a threaded metallic sleeve which enables a nut to cooperate with the threads to press a ring having an inclined surface over the chamfered ends to deform the housing members, thereby pressing them against the outer conductor of a coaxial cable.
Such known methods for assembling a terminal to a coaxial cable are labor intensive and technique sensitive. These methods require significant manual manipulation of the shielding braid which deforms the outer conductor causing reflection and insertion loss. Crimping also deforms the outer conductor causing reflection and insertion loss. Variation in assembly technique impacts the integrity of the connection and results in variation in performance.
Known coaxial cable connectors commonly include a threaded, cylindrical ground shield which has an axially extending bore. These known ground shields commonly include plating on outside surfaces and on inside surfaces which define the bore. It is difficult to apply a uniform plating to inside surfaces of such a design. Plating voids thereby result which increase reflection and leakage losses.
As the use of high frequency systems such as digital satellite radio, Global Positioning Systems (GPS), cell phones, and mobile television continues to grow there is an increasing need for coaxial cable connectors which meet the radio frequency performance needs of such systems, offer consistent performance, and are easy and inexpensive to assemble.
The present invention provides advantages and alternatives over the prior art by providing a terminal for connection to a coaxial cable, together with a ground shield and a method for assembling the ground shield to a coaxial cable. The terminal comprises a two-piece ground shield capable of being connected to an outer conductor of the cable without the need to deform the outer conductor either by dressing it over the ground shield or by crimping it to the ground shield. Importantly, the terminal is capable of being used to carry high frequency signals such as those used by communication and entertainment systems in automotive and other applications.
According to a preferred embodiment of the present invention disclosed herein, a terminal assembly for a coaxial cable is provided which includes a ground shield with surfaces defining an internal passageway In the preferred embodiment, the ground shield includes a strain relief portion having protruding ribs for engaging an outer dielectric sheath of the coaxial cable and a contact portion for electrically and mechanically bonding an outer conductor of the coaxial cable to the ground shield. As a result, it is not necessary to crimp the outer conductor to the ground shield.
In the preferred embodiment, the ground shield is comprised of two identical shield halves joined at a plane extending along an axis of the internal passageway. Each of the shield halves is constructed using a die cast process enabling strain relief features, internal component retention features, and shapes that optimize radio frequency performance to be easily produced. The two-piece design enables the ground shield to have a uniform plating. Each of the shield halves is easier to reliably plate than a complete shield having a bore.
A method is provided for assembling the ground shield of the present invention to a coaxial cable. The method enables automated assembly of the ground shield to the cable.
These and other features and advantages of the present invention will become apparent from the following brief description of the drawings, detailed description, and appended drawings.